In a poignant signal of a fast-changing media landscape, The Boston Phoenix sent out a short and simple tweet Thursday afternonon: “Thank you Boston. Good night and good luck.”

With that terse dispatch, the ground-breaking, Boston alternative weekly, which only six months ago reinvented itself from tabloid newspaper into glossy magazine, put a final punctuation mark on its announcement that its current issue, dated March 15, will be its last.

Sister publications in Providence and Portland, Maine, will stay in business, but WFNX.com, the Phoenix Media/Communication Corp.’s online radio station, will not continue in its present form, its fate to be decided shortly. The company’s custom publishing unit and MassWeb Printing operation, based in Auburn, Mass., will remain open.

The online edition of the Boston Phoenix, slated to appear March 22, will be its last, too.

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New Yorker staff writer Susan Orlean, one of many prominent journalists whose career started at the Boston Phoenix, said from Los Angeles, “It’s like finding out your college has gone bankrupt and is gone. I am a child of the alt-weekly world and I feel like it has played such an important role in journalism as we know it today.”

Employees at the Phoenix, whose origins date back to 1966, were told of the closings by owner and publisher Stephen M. Mindich at a 2 p.m. meeting Thursday. It is expected that about 40 employees will be let go within the week and another ten or so soon after, according to executive editor Peter Kadzis, who described the general reaction among Phoenix staffers as “shell-shocked.” Several people were crying during the meeting, according to one person who was there.

There had been widespread apprehension about a shutdown on Wednesday, when the meeting was announced, Kadzis said, and staffers in Portland and Providence feared they might also be let go.

“Keeping the Phoenix afloat was costing Stephen more than $1 million a year,” Kadzis calculated. “He’s performed an incredible service to the community, and I don’t think most of the employees here realize how committed he’s been to keeping the paper going.”

The Boston Phoenix’s owner and publisher doesn’t plan a formal bankruptcy filing, but the company has hired, the Gordon Law Firm, in Boston to liquidate the paper’s assets and distribute the proceeds to creditors.

Attorney Stephen F. Gordon, who will oversee the process, estimated the business had $1.2 million in debts, but said it’s likely the assets will fetch significantly less than that. The company’s main asset is roughly $500,000 in promised services and goods the Phoenix received in exchange for advertising, but it’s not clear how much a buyer would pay for the bartered goods. It also has some intellectual property and furniture.

Gordon first plans to pay any taxes, employee wages, and fees related to the liquidation process and then distribute any remaining cash evenly to other creditors. In addition to employees, the company owes money to roughly 40 other creditors, including law firms, accountants, utilities, landlords and suppliers.

According to Kadzis, the switch from tabloid to glossy last October won favor with readers and local advertisers. At the time, Phoenix Editor Carly Carioli said, “It’s not a surprise this has been portrayed as the sky is falling, but that’s not what it feels like to us here.”

Six months later, however, the end came. There were not enough national advertisers to make the glossy weekly economically viable. Providence and Portland have been better able to sustain themselves with local advertising, Kadzis noted.

Last spring, Phoenix Communications sold its interest in other media properties, notably the Spanish-language paper El Planeta and terrestrial radio station WFNX 101.7 FM.

News of the Phoenix’s closure only six months after the format change blindsided even alternative media insiders.

“It was shocking to me; I was not expecting this at all,” said Tiffany Shackelford, executive director of the Association of Alternative Newsmedia in Washington. “My understanding was that the new format was successful and that the glossy was starting to attract national advertisers.”

Local media critic Dan Kennedy, a former Phoenix staff writer, wrote on his “Media Nation” blog that he was “not even going to try to write a real post about this today.”

Orlean, too, was “reeling,” saying she received her “education as a writer” at the Phoenix when she worked there in the 1980s. Others who started at the paper include Joe Klein, Sidney Blumenthal, Janet Maslin and David Denby.

The weekly’s closing, Orlean continued, “Removes one more venue for a certain kind of writing that I know was very important to me.”

Shackelford said that despite the loss of a “storied brand” like the Boston Phoenix, the alternative news industry remains healthy.

“Many of our papers are actually improving circulation,” she said. “This [closure] is not indicative of the larger health of the industry. I don’t think any of our other publications are in danger of closing.”

In general, however, alternative newspapers in large markets, like Boston, are not flourishing at the level of their counterparts in smaller, less competitive cities, Shackelford added. It makes sense, she said, that the Portland Phoenix will remain open, as will the Providence Phoenix, which plans to add four full-time reporters.

The question that worries Shackelford is whether another news outlet will “speak truth to power” in the way the Boston Phoenix, which started as Boston After Dark, did for 47 years.

Mindich, in his statement to employees, said he was “extremely proud, as all of you should be, of the highest standards of journalism we have set and maintained throughout the decades in all of our areas of coverage and the important role we have played in driving political and socially progressive and responsible agendas; in covering the worlds of arts and entertainment, food and fashion – always with a critical view, while at the same time promoting their enormous importance in maintaining a healthy society; and in advocating for the recognition and acceptance of a wide range of lifestyles that are so valuable for a vibrant society.”

He thanked all who’ve worked for the Phoenix over the decades _ “our staff has been our soul, “ he averred _ and all the readers and advertisers who’ve supported the publication.